Recently, a liquid crystal display (LCD), one of the flat panel display devices, has been most widely used in various applications including laptops, TVs, and automotive GPS systems. Its demand has grown consistently owing to its advantages such as low electrical power consumption, slimness, and high resolution. An LCD, however, is a passive device, which is not capable of emitting light by itself. Hence, it has to use a backlight unit (BLU) as a light source, for which various optical films are employed. Particularly, white porous polyester films have commonly been used as a reflector sheet for a BLU.
There have been disclosed a number of methods available for preparing white porous polyester films. For example, a polyester film can be prepared by mixing a polyester resin with a foaming agent or mixing a polyester resin with a polyolefin resin to form microvoids on the surface and inside of the film. In the method of using a foaming agent, however, it is difficult to uniformly control the size of microvoids if the processing temperature is not carefully adjusted. In the method of mixing a polyester resin with a polyolefin resin, it is difficult to uniformly control the size of microvoids since the polyolefin resin is incompatible with the polyester resin, resulting in film breakage during the drawing step of the film, as well as unsatisfactory whiteness, reflectance and hiding power.
In order to resolve such problems, it has been attempted to employ various compatibilizers. But it has turned out unsatisfactory in commercial processes due to such problems as low heat stability of the compatibilizers and undesired migration thereof to the film surface.
For the purpose of addressing said problems, in the process disclosed in Korean Patent No. 0215496, inorganic particles are added to a mixture of polyester and polyolefin resins so as to secure sufficient porosity and void size.
Further, Korean Laid-open Patent Publication No. 2011-19614 discloses a white porous polyester film comprising a polyester resin, inorganic particles, and a crystalline polymer resin that is incompatible with the polyester resin. However, the size of incompatible crystalline polymer is not uniform in the dispersed phase, which thus forms irregular voids in terms of the size and hinders stable operation of the process. Also, the number of voids per unit area is not enough to produce a film having desirable and uniform reflectance.